Holographic memories have long promised a means of storing large amounts of information in a compact medium. Recent advances in information technology have emphasized the storage of pictorial images rather than purely digital information. Furthermore, optical computing systems have been proposed which parallel process a large number of optical input data. Holographic memories have natural advantages for storage of such optical data. Nonetheless, the problems of holographic memories have prevented their widespread use.
It is generally desirable in a large memory that different parts of the memory space be randomly accessible. There have been many proposals for spatial multiplexing of holographic memories that allows such random accessibility. In such spatial multiplexing, a holographic recording medium (a photographic plate may be taken as a simple example) is divided into a spatial array of subholograms which are holographically recorded with separate pages of information. In the read-out procedure, a coherent beam of light, such as from a laser, is directed to the desired subhologram so as to read out a desired page of information. Such spatial multiplexing systems are disclosed, for example, by Magill et al in U.S. Pat. No. 3,651,498, by Ando et al in U.S. Pat. No. 3,841,729 and by Nagao in U.S. Pat. No. 4,094,011. Reynolds et al in U.S. Pat. No. 3,542,448 disclose a spatial multiplexing system in which a 100.times.100 matrix array of injection lasers of indeterminate structure are used as reference beams. One of the lasers is selected to illuminate a respective area of a recording medium while the recording medium is being simultaneously irradiated by an information beam produced by a 1000.times.1000 array of injection lasers. The problem of the required coherence between lasers is not addressed by Reynolds et al. On read-out, one of the 100.times.100 lasers is activated to irradiate the selected area.
In spite of the great interest in spatial multiplexing, it is not considered to fully exploit the potential of holographic memories. Holographic memories are typically configured to store the spatial Fourier transforms of information. Therefore, a spatially small defect in the recording medium, while affecting a large quantity of information, may be small enough as to not render any bit of information unreadable. That is, holographic memories are to some extent fault tolerant. However, with spatial multiplexing a recording defect which may be small for the entire recording medium may be large enough on the subhologram as to render it unreadable. Furthermore, there is a problem with cross-talk between neighboring subholograms. Lastly, prior art spatially multiplexed holographic memories have suffered from the problem of optics that need to be critically aligned in order to achieve coherent beams over different areas of the recording medium.
A further holographic technique, referred to as angular multiplexing, overcomes many of the disadvantages of spatial multiplexing. A hologram is recorded by the constructive interference between an image beam (containing the image to be recorded) and a reference beam. In angular multiplexing, multiple holographic images may be recorded on the same area of the recording medium if there are differences in the writing angle .theta..sub.W between the object beam and the reference beam. On reconstruction (read-out), the corresponding reading angle .theta..sub.R of the reference beam on the recording medium will determine which of the images will be constructively imaged on the image plane. There may be a two-dimensional distribution of angular differences, allowing a large number of independently recorded and accessed images. In angular multiplexing, every image is spread out over the entire recording medium to improve fault tolerance, cross-talk is reduced, and thus the recording density on the holographic memory is increased. Gaylord discloses a type of angular multiplexing on pages 387-393 in the textbook "Handbook of Optical Holography" (ed., H. J. Caulfield, Academic Press, 1979).
Nonetheless, angularly multiplexed holographic memories have not found widespread application. All known prior art systems have used a single laser source to produce the coherent read-out beam. Then some sort of beam steering is required to deflect the beam at the selected angle with respect to the holographic recording medium. Beam steerers based on mechanically movable optics have inherent limitations and will not be discussed. Two of the more advanced types of beam steerers are an XY beam deflector and a spatial light modulator.
An XY beam deflector consists of two acousto-optic or electro-optic deflectors with associated anamorphic optical components. The deflectors act as diffraction gratings with an electrically controllable period. The laser beam is initially expanded into a large plane wave and then directed by the XY beam deflector at the desired two-dimensional angle with respect to the recording medium. A further set of optics image the light diffracted from the holographic recording medium onto an image plane, for example, a two-dimensional photo-detector array. This system is, however, bulky and complicated. The capacity-speed product is inherently limited by the frequency bandwidth of the deflectors. For example, an acousto-optic deflector with a frequency bandwidth .DELTA.f=100 MHz allows only 50 resolvable directions into which the beam can be directed in less than 1 .mu.sec.
A spatial light modulator (SLM) is a device which receives a plane wave laser beam and passes only those spatial portions, which are coherent with each other, that have been selected by a separate non-coherent image shone thereupon. Paek et al have proposed the use of a SLM in an associative holographic memory in a technical article entitled "Holographic associative memory for word-break recognition" appearing in Optics Letters, volume 14, 1989 at pages 205-207. A similar use is disclosed in a technical article by Bleha et al entitled "Application of the liquid crystal light valve to real-time optical data processing" appearing in Optical Engineering, volume 17, 1978 at pages 371-384. Although an SLM presents conceptual advantages for angularly multiplexed holographic memories, they are bulky and expensive. Further, the relatively poor quality of available SLMs cause poor reconstructed image quality because of their limited resolution and poor contrast.